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ALGARVE, Portugal — A new two-piece instrument provides more precise measurement for determining anterior chamber intraocular implant diameter than older estimating methods. It is manual, simple and precise, as demonstrated by its inventor, Prof. Manfred Tetz, MD, of Berlin University Eye Clinic, here at the annual meeting of the European Society of Cataract and Refractive Surgeons.

The problem of correct sizing is crucial when implanting angle-supported lenses. Incorrect sizing may lead to angle erosion, pupil ovalization and lens instability.

“Basically, we want a lens that is at risk for minimum contact with the corneal endothelium, the iris and the anterior chamber angle,” Prof. Tetz said.

“In the latest modifications of the Baikoff NuVita [Bausch & Lomb Surgical, Claremont, U.S.A.], which are based on the Kelman Multiflex 4-point fixation design, most aspects have been taken into consideration,” Dr. Tetz said. “Nevertheless, problems and complications can only be overcome if the lens is correctly sized to fit the individual eye.”

Anterior chamber depth

An accurate measurement of the anterior chamber dimensions should take into consideration the three axes: the anterior chamber depth (z), the horizontal axis of the anterior chamber meridian (x; measured from angle to angle) and the vertical axis of the anterior chamber meridian (y).

With the Orbscan (Bausch & Lomb Surgical), the anterior chamber depth can be measured at different points.

“From the Orbscan, which we have been using for some years on myopic eyes, we learned that there are no rules, and that each eye is different,” Prof. Tetz said.

The Orbscan map of a –15 D myopic patient shows a central depth of 2.68 mm and a peripheral depth of 2.89 mm. Other maps show how the central depth can vary from the 3.33 mm of a –8 D, to the 2.68 mm of a –15 D, to the 3.44 of a –11 D. The difference in depth between center and periphery also is variable.

“Central measurement of the anterior chamber depth is almost irrelevant,” Prof. Tetz said. “It is far more important to take the measurements in the eccentricity, where the lens of the phakic or pseudophakic implant may be thickest, and where there is, therefore, a great-er possibility of contact with the endothelium. This is usually at 2.5 to 2.7 mm eccentricity. When you look at these data, it is amazing to see that although some myopic eyes are narrower in the middle than in the periphery, others reveal exactly the opposite.

This is one of the reasons Prof. Tetz recommends inserting the phakic anterior chamber lens horizon- tally rather than vertically. In this way, the loop-optic junction is more likely to be in the deepest part of the chamber. This applies even more to hyperopes, who have shallower chambers. Horizontal placement of the IOL renders the measurement of the y axis irrelevant.

Precise angle-to-angle measurement

As for the horizontal axis (angle to angle), no more efficient means has been developed so far than the white-to-white measurement, which is well known to be inaccurate and often misleading, Prof. Tetz said.

Ovalization of the pupil is the most common consequence of inaccurate angle-to-angle measurement, he added. This phenomenon, which Dr. Tetz said from his personal experience and from results reported at meetings by his European colleagues, had been reported in 10% to 15% of NuVita ZB5M implants (the second and previous generation of lens). Dr. Tetz said ovalization is due to the constant traction and scar tissue formation caused by the footplate of wrongly sized lenses, he explained.

“How can you explain to young patients that they may end up with cat-like eyes?” Prof. Tetz said. “I’ve always thought that the inaccuracy of our methods for sizing angle supported lenses is absolutely unacceptable in this day and age, when the technology of refractive surgery provides measurements in the range of microns for other procedures.”

To overcome this problem, Prof. Tetz created a two-piece device that provides accurate measurements of the angle-to-angle distance with simple intraoperative maneuvers. The first piece is a centration ring with an outer diameter of 11 mm. The ring is placed on the eye, and the center of the cornea is marked with the tip of a 27-gauge cannula. The second piece looks like a thin-angled spatula, with a scale on the handle and a double end.

“This is exactly the footplate I want to use in my implant,” Prof. Tetz said. “I insert the instrument through the incision made for the lens, after having filled the chamber with viscoelastic. I push the instrument forward until its endings come into contact with the angle, in exactly the same way the lens footplates will do. Keeping the eye horizontally under the microscope, the central mark of the cornea is aligned with the scale on the spatula, thus giving the radius of the anterior chamber. If you double the distance, you will have the exact angle-to-angle diameter and choose the IOL size accordingly.”

Prof. Tetz used this instrument in 20 eyes, and in more than 50% of cases he implanted a lens of different size from what he would have chosen taking white-to-white measurement only.

“I am now much happier with my results. The correct sizing obtained with this simple aid can now easily be judged by the perfectly round shape the pupil maintains after surgery and by the comfortable fit and perfect stability of the lens,” he said.

Editor’s note: Dr. Antonio Marinho reported a rate of pupil ovalization of 10% with the previous generation ZB5M lens. Only two cases of ovalization were seen in his 2-year follow-up of 60 eyes, and these did not progress. He said it is likely pupil ovalization is caused by incorrect sizing of the IOL; long-term data will confirm if this has been resolved by the redesigned NuVita.

Accurate measurement of anterior chamber dimensions should include three axes: anterior chamber depth (z), horizontal axis of the anterior chamber (x) and vertical axis of the anterior chamber (y).	Anterior chamber depth can vary from central to peripheral measurements.
In young patients with myopia, peripheral measurements are the same above, below, nasally and temporally.	In older patients, there is a tendency for the chamber to be shallower superiorly.
A two-piece set of devices provides accurate measurements of the angle-to-angle distance with simple intraoperative maneuvers.	One piece looks like a thin, angled spatula, with a scale on the handle and a forked ending reproducing the shape of a Kelman Multiflex IOL footplate.

For Your Information:

• Prof. Manfred Tetz, MD, can be reached at Klinik für Augenheilkunde – Universität zu Berlin, Augustenburger Platz 1, D 13353 Berlin, Germany; +(49) 30-45054011; fax: +(49) 30-45054911; e-mail: mtetz@charite.de. Dr. Tetz has a direct financial interest in the Anterior Chamber Measuring Device. He is not a paid consultant for any companies mentioned.

• For more information on the Anterior Chamber Measuring Device, contact Bioshape, Fregestr 87, 12159 Berlin, Germany; e-mail: info@bioshape.com; Web site: www.bioshape.com.